1. Field of the Invention
The present invention relates to a backlight unit used in liquid crystal display (LCD) televisions, LCD monitors and the like, and a cold cathode fluorescent lamp (CCFL) forming the main light source of the backlight unit, and particularly to backlight units and CCFLs for use in large LCD TVs.
2. Related Art
In recent years, there has been increasing demand for LCD televisions, LCD monitors and the like, as well as for backlight units used in these devices. In terms of backlight units, there is the edge-lit type used in LCD monitors for PCs and small LCD TVs etc., and the direct-lit type used in large LCD TVs etc.
Generally, a direct-lit backlight unit includes a plurality of CCFLS, a diffusion plate disposed on a light-emission side of the backlight unit, and a reflective plate disposed on a light-reflecting side of the backlight unit. A diffusion sheet and a lens sheet are disposed on the light-emission side of the diffusion plate, and the LCD panel of an LCD TV is disposed on the light-emission side of the lens sheet. These plates and sheets are generally made of resin.
CCFLs are ideal for reducing the depth and weight of backlight units given the small-diameter, thin-walled construction of the light-emitting tubes, which are formed using borosilicate glass. Owning to its high melting point in comparison with soda lime glass, borosilicateglass does not deform at the baking temperature of the phosphors, even when used in small-diameter, thin-walled light-emitting tubes, and its high mechanical strength makes it robust.
The borosilicate glass (also called hard glass or heat-resistant glass) referred to here includes boron oxide (B2O3) at 9 wt % or greater, this being the largest component after silicon dioxide (SiO2). Soda lime glass includes sodium oxide (Na2O), calcium oxide (CaO) and the like as primary components, with only a small amount of boron oxide present. Note that materials such as kalium oxide (K2O) are sometimes used in soda lime glass in place of sodium oxide (Na2O).
CCFLs generate visible light by using ultraviolet light (hereinafter “UV light” or simply “UV”) irradiated by discharges in a low-pressure mercury vapor to excite phosphors. The line spectrum of the irradiated UV light peaks at wavelengths including 254 nm, 313 nm, and 365 nm.
Some of the UV light irradiated from the discharges, however, passes through the phosphors and leaks to the outside of the light-emitting tube, where it is irradiated onto the diffusion and reflection plates of the backlight unit. UV light causes resin members such as the diffusion and reflection plates to deteriorate and discolor, resulting in a loss of transparency and transmissivity. The surface brightness of the backlight unit is consequently reduced after a relatively short operating time.
To block UV light passing through the phosphors with the light-emitting tube so as to prevent the leaking of UV light, the light-emitting tube is doped with a UV absorbent. That is, a UV absorbent is included in the light-emitting tube as one of the components constituting the borosilicate glass. Titanium oxide (TiO2), cerium oxide (CeO2) and zinc oxide (ZnO2) are examples of UV absorbents, with a typical light-emitting tube being doped with approximately 0.5 wt % titanium. Doping the light-emitting tube with titanium at approximately 0.5 wt % makes it possible to almost completely block 254 nm UV light, which has the most detrimental effect on the surface brightness of the backlight unit. On the other hand, 313 nm and 365 nm UV light cannot be sufficiently blocked, even by the doping of titanium at approximately 0.5 wt %.
Japanese Published Patent Application No. 2003-167250 discloses a backlight unit having little reduction in surface brightness because of UV light being blocked over an extremely wide wavelength region. FIG. 1 is a schematic view showing a main section of a CCFL according to the above application. In CCFL 30, a UV absorbent layer 33 (0.02–0.5 mg/cm3) is formed between the inner surface of a light-emitting tube 31 and a phosphor layer 32, in addition to the borosilicateglass of tube 31 being doped with 0.1–0.8 wt % of a UV absorbent.
Japanese Published Patent Application No. 2003-73142 discloses a lighting glass with little excess UV light and excellent heat treatability, that is targeted for use in fluorescent lights, incandescent bulbs and the like. The disclosed lighting glass is soda lime glass doped with 0.05–3.0 wt % titanium oxide or cerium oxide as the UV absorbent.
In recent years LCD TVs have been increasing in size, with large LCD TVs having screens in excess of 17 inches now in widespread use. Large LCD TVs employ PC (polycarbonate) resin diffusion plates rather than the acrylic resin diffusion plates used by small LCD TVs.
The high absorbency of acrylic resin means that acrylic resin diffusion plates easily warp due to absorption, and given that the measurement error from warping also increases with increases in the size of the diffusion plate, problems readily occur in the design of backlight units. PC resin, on the other hand, has excellent anti-absorbency, mechanical strength, heat resistance and light transmittance characteristics, which means that the above problems do not arise since there is hardly any warping due to absorption with diffusion plates made of PC resin.
However, a problem with the PC resin diffusion plates employed in large LCD TVs having screens in excess of 17 inches was that the deterioration and discoloration due to UV light occurred more readily than with the acrylic resin diffusion plates employed in small LCD TVs. As such, the inventors identified the marked deterioration/discoloration of PC resin diffusion plates due to 313 nm UV light, which was not really a problem with the acrylic resin diffusion plates.
Also, the difficultly, with CCFL 30 in the above Published Patent Application No. 2003-167250, of adhering the UV absorbent evenly to the inner surface of light-emitting tube 31 due to the small diameter of the tube, meant that the adhered amount of UV absorbent fluctuated in a lengthwise direction of the tube. Light-emitting tube 31 was particularly susceptible to fluctuations in the adhered amount of UV absorbent because of its long length. Consequently, UV light was insufficiently blocked in areas where the adhered amount of UV absorbent was reduced due to the fluctuations.
In addition, the need for a process of adhering the UV absorbent to the inner surface of light-emitting tube 31 in the manufacture of CCFL 30 caused a hike in production costs.
On the other hand, light-emitting tubes for CCFLs could not be manufactured with the lighting glass of the above Published Patent Application No. 2003-73142 because of the use of soda lime glass. As described above, soda lime glass has a lower melting point than borosilicate glass, which means that the light-emitting tube readily deforms at the baking temperature of the phosphors, and also the low mechanical strength of soda lime glass makes the light-emitting tube susceptible to breakage.